I don't think it is possible. Those are called modifier keys and you can't only use them in shortcut, because the system wouldn't know whether it should interpret ⌘⇧ or wait you add another key like S to interpret ⌘⇧S. That's why you can't use only modifier keys in keyboard shortcuts.

However, some software (like Alfred) allows you to use a fast double press on a modifier key as a shortcut. Apple uses this trick as well since Mountain Lion with the fn key in order to activate voice dictation.

I would hesitate to do this. What would happen if, say, you press Command-[any key], and press the Command key just a few milliseconds before the other? Would the OS interpret that as a Command-key press and perform an action? Most likely, the inability to set these keys as shortcuts on their own is for that reason.